Microscale patterning and articles formed thereby

ABSTRACT

The present invention is directed to a lithographic method and apparatus for creating micrometer, more particularly sub-micrometer patterns in a thin film coated on a substrate. The present invention utilizes the self-formation of periodic, supramolecular (micrometer scale) pillar arrays in a thin melt to form the patterns. The self-formation was induced by placing a second plate or mask a distance above the polymer film. The pillars bridge the plate and the mask, having a height equal to the plate-mask separation (preferably 2-7 times that of the film&#39;s initial thickness). If the surface of the mask has a protruding pattern (e.g., a triangle or rectangle), the pillar array is formed with the edge of the pillar array aligned to the boundary of the mask pattern.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This is a continuation of application U.S. Ser. No. 09/807,266,filed on April 9, 2001, which is a national phase filing based onInternational Application No. PCT/US99/23717, filed on Oct. 8, 1999,which claimed the benefit of priority from U.S. Provisional PatentApplication Ser. No. 60/103,790, filed on Oct. 9, 1998.

GOVERNMENT INTEREST

[0002] This invention was made with Government support under ContractNo. 341-6086 awarded by DARPA. The Government has certain rights in thisinvention.

FIELD OF THE INVENTION

[0003] The present invention relates generally to forming patterns on orin a surface material, assemblies used therefor, and articles formedthereby. More specifically, the present invention relates to microscalepatterning and/or lithography. Microscale patterning and microscalelithography have a broad spectrum of applications, e.g. in theproduction of integrated circuits, microdevices, and the like. Thepatterns formed can be utilized to perform an array of functions,including electrical, magnetic, optical, chemical and/or biologicalfunctions.

BACKGROUND

[0004] One of the key processing methods in fabrication ofsemiconductors, integrated electrical circuits, integrated optical,magnetic, and mechanical circuits and microdevices is forming very smallpatterns.

[0005] Lithography is often used to create a pattern in a thin filmcarried on a substrate so that, in subsequent process steps, the patternwill be replicated in the substrate or in another material which isadded onto the substrate. One purpose the thin film satisfies isprotecting a part of the substrate so that in subsequent replicationsteps, the unprotected portion can be selectively etched or patterned.Thus, the thin film is often referred to as a resist.

[0006] A typical lithography process for the integrated circuitsfabrication involves exposing a resist with a beam of energeticparticles which are electrons, or photons, or ions, by either passing aflood beam through a mask or scanning a focused beam. The particle beamchanges the chemical structure of the exposed area of the film, so thatwhen immersed in a developer, either the exposed area or the unexposedarea of the resist will be removed to recreate the pattern or obverse ofthe pattern, of the mask. A limitation on this type of lithography isthat the resolution of the image being formed is limited by thewavelength of the particles, the particle scattering in the resist, thesubstrate, and the properties of the resist. Although pattern sizesgreater than 200 nm can be achieved by photolithography, and patternsizes in the range of 30 nm to 200 nm can be achieved utilizing electronbeam lithography, these methods are resource intensity and suffer fromlow resolution.

[0007] U.S. Pat. No. 5,772,905 describes a method and apparatus forperforming ultra-fine line lithography wherein a layer of thin film isdeposited upon a surface of a substrate and a mold having at least oneprotruding feature and a recess is pressed into the thin film.

[0008] An alternative strategy to those described above is to use a“naturally occurring” or “self-assembly” structure as a template forsubsequent parallel fabrication. For example, U.S. Pat. No. 4,407,695and U.S. Pat. No. 4,801,476 describe a spin coating technique to prepareclose-packed monolayers or colloidal polystyrene spheres with diametersof typically 0.1-10 microns on solid substrates. The pattern is thenreplicated by a variety of techniques, including evaporation through theinterstices, ion milling of the spheres and/or the substrates, andrelated techniques. Highly ordered biologically membranes (“S-layers”)have also been suggested as starting points for fabrication. Closepacked bundles of cylindrical glass fibers, which could be repeatedlydrawn and repacked to reduce the diameters and lattice constant havealso been used. Block copolymer films have been suggested for use aslithography masks wherein micelles of the copolymer which form on thesurface of a water bath are subsequently picked up on a substrate.

[0009] To date, the focus of “self-assembly” has been primarily oneither phase separation of a polymer blend, of di-block copolymers, orof local modification of surface chemistry (i.e., chemical lithography).In self-assembly by phase separation, the periodic structures aremultidomain, and their orientation and locations are uncontrollable andrandom. A long-sought after goal in self-assembly is precise control ofthe orientation and location of a self-assembled polymer structure.

[0010] There is an ongoing need to produce progressively smaller patternsize. There also exist a need to develop low-cost technologies formass-producing microscale and sub-micron (e.g. nanometer) structures.Microscale, indeed nanoscale and smaller, pattern technology will havean enormous impact in many areas of engineering and science. Both thefuture of semiconductor integrated circuits and the commercialization ofmany innovative electrical, optical, magnetic, and mechanicalmicrodevices that are far superior to current devices will depend onsuch technology.

SUMMARY OF THE INVENTION

[0011] Technologically, self-assembly promises not only low-cost andhigh-throughput, but also other advantages in patterningmicrostructures, which may be unavailable in conventional lithography.

[0012] The present invention is generally directed to the formation ofpatterns in a material through deformation induced by a mask placedabove a material, as well as assemblies used therefor, and productsformed thereby. An important aspect of the present invention is novelmethod, referred to herein as “lithographically-induced self-assembly”(LISA). In this process a mask is used to induce and controlself-assembly of a deformable surface, preferably a thin film into apre-determined pattern. One advantage of the present invention isrelatively accurate control of the lateral location and orientation of aself-assembled structure. Preferably, a substantially uniform, film iscast on a substrate. A mask, preferably with protruding patternsrepresenting the pattern to be formed in or on the film, is placed abovethe film, but physically separated from the film by a gap. The mask, thefilm, and the substrate are manipulated, if necessary, to render thefilm deformable. For example in the case of a polymer, the polymer filmmay be heated to a temperature above the polymer's glass transitiontemperature and then cooled down to room temperature. During theheat-cool cycle, the initially flat film assembles into discreteperiodic pillar arrays. The pillars, formed by rising against thegravitational force and surface tension, bridge the two plates to formperiodic pillar arrays. The pillars generally have a height equal to theplate-mask separation. Moreover, if the surface of the mask has aprotruding pattern, the pillar array is generally formed only under theprotruding pattern with the edge of the array generally aligned to theboundary of the mask pattern. After the pillar formation, due to aconstant polymer volume, there is little polymer left in the areabetween pillars. The shape and size of the mask pattern can be used todetermine the pillar array's lattice structure. The location of eachpillar can be controlled by the patterns on the mask. This process canbe used repeatedly to demagnify the self-assembled pattern size. Thisdemagnification permits a self-assembled structure to have a sizesmaller than that of the mask pattern(s). If the demagnification is usedrepeatedly, a size much smaller than that by a single self-assemblyprocess can be achieved. This would allow for progressively smallerpattern-mask-patterns to be formed. The basic LISA process can also bemodified to form a non-pillared pattern that is substantially identicalto the features of the mask.

[0013] One embodiment of the present invention is a patterning method ormethod of patterning which comprises depositing a material on asubstrate. The material and substrate may be already formed, and thematerial and substrate may be the same or different. In this case thestep of depositing a material would not be necessary, but rather asurface layer(s) would be selectively manipulated so that apredetermined thickness of surface material is deformable. Thisthickness must be small enough that the mask can interact with thematerial through the separation distance to form a contact therebetween.As is described more fully herein, the thin film or surface layer(s)preferably has a thickness in the range of about 1 nm to about 2,000 nm,more preferably about 10 nm to about 1,000 nm, more preferably about 100nm to about 500 nm and even more preferably about 50 nm to about 250 nm.If the deposited material is deformable at room temperature (e.g., aliquid polymer or polymer dispersion, the material may not need to bedeformed). If a liquid polymer is used, it may be cured (e.g., photocuring) after either pillar formation, usually before removal of themask. For a solid material, it may be necessary to render the materialdeformable, e.g. by heating to a temperature where the material mayflow. Deforming by heat is a preferred route, but the material orsurface layer(s) may also be deformed by, other routes (e.g., chemicalreactions). Heating may occur by any conventional means (e.g., laser,light sources, heat radiating or microwave induction), and the heat maybe pulsed or continuous.

[0014] It is important that the mask be maintained above the material orfilm. A spacer (which may be integrally or non-integrally formed withthe mask) is convenient to this end. However, an assembly may be usedwherein the mask is maintained above said material without resorting toa spacer.

[0015] The substrate can be any number of compositions which are capableof supporting the film, but the present invention has particularapplicability to substrates which are, themselves intended to beprocessed to have patterns formed thereon or therein. The substrate canhave pre-existing relief patterns or be flat.

[0016] The mask can be of any suitable material as described herein. Inmany cases, the mask, will often be very similar in composition to theunderlying substrate. Indeed, it is envisioned to use a suitablypatterned substrate from a previous LISA process in a second or moreLISA process or LISC process. The mask can have any suitable surfacecoating and the protrusion may be formed from a surfactant or othersuitable protruding material (e.g., monolayers or self-assembledmonolayers) with a different surface energy. The protruding pattern maybe of varying heights on the same pattern resulting in like pillars. Ofcourse, any combination of protrusion pattern protrusion coating ormonolayer material pattern may be used to form the relief structure.

[0017] Another embodiment of the present invention is the reliefstructure formed by either or both the LISA and LISC process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The features, aspects, and advantages of the present inventionwill become better understood with regard to the following description,appended claims, and accompanying drawings wherein:

[0019]FIG. 1 schematically illustrates lithographically-inducedself-assembly (LISA): (a) a flat substrate, (b) a thin layer ofdeformable material deposited thereon, (c) a mask with a protrudingpattern a distance above the deformable material; and (d) self-assemblyinto a periodic supramolecular pillar array after a heat-and-cool cycle;

[0020]FIG. 2 is an (a) optical and (b) AFM images of periodic pillarsformed using a mask of a plain flat surface. The pillars have aclosely-packed hexagonal lattice and are multi-domain, covering theentire wafer with a single-domain size of about 50 μm;

[0021]FIG. 3 is an optical micrograph of (a) a protruding trianglepattern on the mask and (b) pillar array formed under the trianglepattern using LISA, and (c) AFM of the pillar array;

[0022]FIG. 4 is an optical and AFM images of the LISA pillar arraysformed under protruding square patterns of a side of (a) 10 μm, (b) 14μm, and (c) 14 μm. The separation between the mask and the substrate (a)430 nm, (b) 280 nm, and (c) 360 nm, respectively;

[0023]FIG. 5 is (a) optical micrograph of a protruding line patternspelling “PRINCETON” on the mask and (b) AFM image of pillars formedunder the mask pattern;

[0024]FIG. 6 is an AFM image of a pillar array formed under a grid-linemask pattern with each pillar aligned under an intersection of the grid;

[0025]FIG. 7 (a) illustrates schematically lithographically-inducedself-assembly using a surfactant as the pattern: (i) A thin layer ofhomopolymer cast on a flat silicon wafer. (ii) A mask of surfactantpatterns placed a distance above the PMMA film, but separated by aspacer. (iii) During a heat-and-cool cycle, the polymer filmself-assembled into a periodic supramolecular pillar array. (b)Schematic of the experimental setup;

[0026]FIG. 8 shows the observed dynamic behavior of the growth of thefirst pillar under a square mask pattern at 120° C. (a) The polymer wasfeatureless before heating the system. The beginning of 120° C. was setas time zero. (b)-(f) At 120° C., the polymer under the corners of themask pattern is being pulled up; (g) the first pillar just touched themask; (h)-(i) the pillar expanded to its final size;

[0027]FIG. 9 shows the observed dynamic behavior of the growth of anarray formed under the square mask pattern at 120° C. from the firstpillar to the last pillar. The pillars were formed in an orderly manner,one by one, first under the corners of the mask pattern, then along theedges, later new corners and new edges, until the area under the maskpattern was filled with pillars. The completion of the first pillar wasset as time zero;

[0028]FIG. 10 is the atomic force image of the same LISA pillar array(5×5 pillars) as in FIG. 8 and 9. The pillars with a flat top have aheight, diameter, and period of 310 nm, 5 μm, and 9 μm, respectively Thearray has a simple cubic lattice;

[0029]FIG. 11 schematically illustrates a proposed model for LISA: (a)surface roughening, (b) roughening enhancement due to a long rangeattractive force, (c) pillar formation, and (d) self-organization;

[0030]FIG. 12 schematically illustrates lithographically inducedself-construction (“LISC”) schematic of LISC: (a) a thin polymer filmcast on a flat substrate (e.g. silicon), (b) a mask with protrudingpatterns placed a distance above the polymer film, (c) during aheat-and-cool cycle, the polymer film self-constructs into a mesa undera mask protrusion. The mesa has a lateral dimension identical to that ofthe mask protrusion, a height equal to the distance between the mask andthe substrate, and a steep side wall;

[0031]FIG. 13 shows (a) optical image of protruded pattern of“PRINCETON” on the mask, and (b) AFM image of the LISC patterns formedunderneath the mask pattern The LISC pattern duplicates the lateraldimension of the mask pattern.

[0032]FIG. 14 illustrates Self-Assembly (SALSA) of a Random-AccessElectronic Device.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0033] It is to be understood that the invention is not limited to aspecific article of manufacture or technique described herein, and maybe implemented in any appropriate assembly or process. Additionally, itshould be understood that the invention is not limited to any particularmaterial or substrate. As is described herein, a variety of types ofmaterials and/or substrate may be used.

[0034] As described herein, placing a mask a distance above a surface,preferably a thin deformable surface results in the formation of aself-assembled periodic supramolecular array of pillars during aheat-and-cool cycle. The pillars are formed in the area under theprotruding mask pattern and are normal to the substrate. The pillarsgenerally extend all the way to the mask (or to protrusion patterns onthe mask). The location of each pillar as well as the size, shape, andlattice structure of the array can be controlled by the patterns on themask with a great deal of precision. The period and diameter of thepillars also can be controlled, depending, for example, on the molecularweight of the polymer Both the period and diameter generally becomesmaller as the lower molecular weight of the polymer is lowered.Although not intending to be bound by theory, it appears the LISAprocess involves a delicate interplay of surface-roughening, long-rangeVan der Waals forces between the surface and the mask, surface meltflow, wetting properties of both mask and substrate, trapping oftriple-phase lines, and balance of attractive and repulsive forces. LISAis extendable to other materials such as semiconductors, metals, andbiological materials. The application of LISA and related technologiesdescribed herein opens up numerous applications in microelectronics,information storage, new materials, biology, and chemistry.

[0035] Turning with specificity to the figures, FIG. 1 schematicallyillustrates the lithographically induced self-assembly of the presentinvention. Onto the substrate 31 is layered a material 33, which, in thepreferred embodiment, is a thin layer of a homopolymer, preferablypolymethyl-methacrylate (PMMA). The PMMA was first spun on substrate 31,in this case, a silicon wafer having a substantially plain flat surface,followed by baking at 80° C. to drive out the solvent. Next a mask 35,typically made of silicon dioxide, with a protruding pattern 37 on itssurface that faces a deformable material 33 is placed above the PMMAfilm 33. As is shown in FIG. 1, the mask 35 is separated, using a spacer39, from the PMMA by several hundred nanometers. The distance betweenthe protrusion 37 and film 33 is preferably in the range of about 10 nmto about 1000 nm, more preferable 50 nm to 800 nm, and never morepreferably that about 100 nm to about 700 nm. The spacer 39 may beeither integrally formed with the mask or a separate element (see e.g.FIG. 12).

[0036] Pattern forming assembly 41 was heated from room temperature to atemperature above the glass transition temperature of the PMMA film 33,and then cooled back to room temperature. During the heat-cool cycle, apress or chuck 51 (shown in FIG. 8b) was used to hold the substrate 31,the spacer 39, and the mask 35 (i.e. the pattern forming assembly) intheir respective positions, thereby preventing substantial relativemovement and maintaining the mask-substrate separation constant. Theopen space between the initial PMMA film 33 and the mask 35 gives thePMMA film 33 freedom to deform three-dimensionally. Preferably, thesubstrate 31 is wet to the deformable material 33. In the case of asilicon substrate, the silicon substrate surface preferably has a thinlayer of native oxide, making it a high energy surface and wet to a PMMAmelt. In a preferred embodiment of the present invention, the masksurface 47 has a monolayer surfactant, making it a low energy surfaceand non-wet to a PMMA melt. The heat-cool cycling was performed eitherin atmosphere or a vacuum of 0.3 torr, which has little effects on thefinal results. The height of the protruding patterns 37 on the mask 35is typically micro scale and generally in the range of about 50 nm toabout 500 nm (in this example 150 nm).

[0037] It was observed that without a mask placed on the top, after aheat-cool cycle, the PMMA film 33 remains flat and featureless. But,with a mask 35 placed a certain distance above the surface of the PMMAfilm 33, after the same heat-cool cycle, the initially flat PMMA film 33became self-assembled into periodic supramolecular pillars 49 shown inFIG. 1. The pillars 49 were formed only under the protruding patterns 37of the mask 35, but not under the recessed areas of the mask 35. Thepillars 49 are normal to the substrate 31 and extend all the way fromthe substrate 31 to the mask 35, making their height generally equal tothe initial separation 43 between the substrate 31 and the surface ofthe mask 35. The location of each pillar 49 as well as the size, shapeand lattice structure of the array is determined by the pattern 37 onthe mask 35.

[0038] A variety of masks having protruding patterns were made. Maskswere formed in a variety of shapes, such as triangles, rectangles,squares, lines, and grids. As can be seen in FIG. 2, for a mask withoutany pattern (i.e., a plain flat surface) placed 165 nm above the surfaceof the PMMA film, the original flat PMMA film became, after a heat-coolcycle, periodic PMMA 15 pillar arrays with a close-packed hexagonalstructure of 3.4 μm period, 2.7 μm pillar diameter, and 260 nm height.The optical images showed that the array of pillars 49 are multidomainand everywhere over the entire sample. The average size of a singledomain is about 50 μm (i.e.—15 periods). The atomic force microscopy(AFM) showed that the top of each pillar 49 is flat (due to contact withthe mask) and the sidewall is quite vertical (due to the tip sizeeffect, the AFM tip is not good for profiling the sidewall).Two-dimensional Fourier transform of the AFM images showed six sharppoints arranged in a hexagonal shape in the k-space, further confirmingthe hexagonal lattice structure of the pillars. The initial PMMAthickness was about 95 nm. The substrate and the mask were heated to130° C. and were held together for 20 min by a pressure of 300 psi. Thenthey were cooled down for 10 min to room temperature before beingseparated.

[0039] As can be seen in FIG. 3, for a protruding triangle pattern 67 onthe mask, a single-domain PMMA pillar 79 array of a close-packedhexagonal lattice is formed under the mask pattern 67. Both optical andAFM images show that the shape and size of the pillar 79 array areidentical to that of the mask pattern, with the pillars 79 on the edgesof the array aligned along the edges of the triangle mask pattern 67.The polymer initially under the recessed area of the mask 35 is depletedafter the LISA process and no pillars 79 are formed under the area. Inthis example initial thickness of the PMMA film 33 was 95 nm. Theinitial separation between the substrate and the mask pattern 67 was 530nm. The LISA pillar array is a close-packed hexagonal structure that hasa periodicity of 3 μm, an average pillar diameter of 1.6 μm. Thetriangle mask pattern 67 has a side of 53 μm; a larger size will makethe pillar array multi-domain.

[0040] As can be seen in FIG. 4, when the protruding patterns on themask are rectangles and squares, the pillar arrays 99 a formed in LISAalso have a shape and size identical to the mask patterns with thepillars 99 a at the edges aligned to the edges of the mask patterns,just as in the case of the triangle mask pattern. However, the latticestructures of the pillar arrays are not hexagonal. It appears that thepillars 99 a on the edges are formed and aligned to the mask patternedges first. Then the other pillars will be formed later according tothe position of the edge pillars. The shape of the mask and the pillarheight determine the final lattice structure of a pillar array. FIG.4(b) and (c) show that a same mask pattern geometry but differentmask-substrate separations lead to two different lattice structures(pillars 99 b and 99 c). Moreover, the pillars at the edges appear tohave a diameter slightly larger than other pillars.

[0041] The size and shape of a number mask used in our experiment aswell as the diameter, period and height of the pillars formed in LISAare summarized in Table 1 below.

[0042] Table 1 below provides a summary of the geometry of the maskpatterns and the LISA Pillars. TABLE 1 Side Pillar Pillar Pillar MaskLength Height Period Diameter Geometry (um) (nm) (um) (um) Plain N/A 2603.4 2.7 Triangle 53 530 3.0 1.6 Rectangle 24 × 12 440 3.3 2.0 Square 10430 3.5 2.0 14 280 3.5 2.1 14 360 3.5 2.0 Line N/A 530 4.8 2.1

[0043] The pillar diameter seems to decrease, with increasing the pillarheight (i.e., the separation between the substrate and the mask), thatcan be understood from the fact that the total polymer volume isconstant. The pillar period was found to vary slightly with differentmask pattern geometry. Further experiments showed that the pillar periodand size depend on the polymer molecular weight. When PMMA of 15Kmolecular weight (made by a vendor different from 2K PMMA) was used, thepillar period and diameter became about 8 μm and about 5 μm,respectively. It was also found that as the heating time could impactpillar formation. An example of high ratio of pillar height to thepillar diameter we achieved is 0.5 (800 nm height and 1.6 μm diameter).

[0044] As can be seen in FIG. 5, if the mask pattern is a protrudingline of a width less than 5 um, a single pillar line will be formed andaligned under the line pattern.

[0045] As can be seen in FIG. 6, if the mask pattern is a grid, a pillar109 is formed and aligned under each intersection of the grid maskpattern. In this case, the pillar 109 period is fixed by the period ofthe grid on the mask.

[0046] In LISA, the plate or mask, when placed a distance above a thinmelt film, preferably a single-homopolymer melt film, causes the polymerfilm, initially flat on another plate, to self-assemble into periodicpillar arrays. The pillars form by raising against the gravitationalforce and the surface tension, bridge the two plates, having a heightequal to the plate-mask separation. If the mask surface has either aprotruding pattern, a surfactant pattern (e.g. with a shape of atriangle or rectangle, etc.), or a combination of the two, a pillararray is formed under the pattern with its boundary aligned to theboundary of the mask pattern and with its lattice structure determinedby the mask pattern geometry.

[0047] To monitor the development of the pattern forming process of LISAin a polymer film, a monitoring assembly 110 was used. The monitoringassembly 110 is shown in FIG. 7b. In this case, the mask was made ofglass, thus allowing for observation through the mask using an opticalmicroscope 112. A CCD camera 114 and video recorder 116 with a timeresolution of 30 millisecond recorded the pattern forming behavior. Thesample consists of 2,000 molecular weight (2K) PMMA polymer 103 cast ona silicon substrate 101 that has a surface that wets the PMMA polymer103. The glass transition temperature of the PMMA was found to be 96° C.The mask 105 has various patterns of a monolayer of self-assembledsurfactant 117. The surfactant 117 was applied to the mask 105 viaphotolithography and a lift-off. The substrate 101 and mask 105 wereseparated by a 220 nm spacer 109 and were held together by a metalholder comprised of a chuck 51 and a screw 53 with the pattern formingassembly interposed therebetween. The entire pattern forming assembly111 was heated by a hot plate 55. The temperature was monitored by athermocouple mounted on the holder.

[0048] The dynamic behavior of a LISA pillar array formation under asquare mask pattern is summarized in FIGS. 8 and 9. The PMMA was 135 nmthick. FIG. 8 shows the growth of the first pillar in the array. Thesample holder was heated from room temperature to 120° C. and maintainedthat temperature for the remainder of the experiment. As can be seen inFIG. 8a, before heating the system, the PMMA was featureless. Thetemperature was increased at a rate of about 10° C./min up to 100° C.and then at 1° C./min after that. Once the temperature exceeded 11 0C, avery faint image showing the outline of a pattern could be observed. Ascan be seen in FIG. 6 this latent image was clearly visible once thesystem reached 120° C. The beginning of 120° C. was chosen as the zerotime reference in FIG. 8. The latent pattern indicates the onset ofpattern formation and is visible because the polymer in that region isseveral tens of nanometers higher than the surrounding film. For asquare mask pattern a latent image formed first under the corners of themask pattern and then the edges. As can be seen in FIG. 8c-e, gradually,the latent image at the corners becomes much more pronounced, indicatingthe growth of polymer pillars at the corners (FIG. 8c-e). It wasobserved that one pillar always grew faster than the rest. In thisparticular example, it took 58 minutes for the first pillar to reach themask. When a pillar just touched the mask, its image became a blackpoint in the center of a bright circle (see FIG. 8g), and then expandedinto a bright dot of 5 μm diameter in 6 seconds (see FIG. 8i). Thissuggests that pillars first grow as a cone-shaped spike in the polymerfilm and then, after touching the mask, reshape into a pillar with aflat top. The mask surface should be a low energy surface to limit thedistance that a pillar can spread.

[0049]FIG. 9 shows the growth behavior from the first pillar to the lastpillar of a LISA array formed under the square mask pattern. The timezero in this figure is set at the completion of the first pillar. Theformation of the second pillar was completed 9 seconds after the firstpillar, in a corner of the mask pattern adjacent to the first pillar(See FIG. 9b). The third pillar was completed 58 seconds later and wasin a corner adjacent to the second pillar (See FIG. 9c). And the fourthpillar was formed at 2 min 59 seconds (See FIG. 9d). After the pillarsat the corners were completed, new pillars started to form at the edgesof the mask pattern (See FIG. 9e-g). A new edge pillar was observed toalways form adjacent to an existing pillar. After the first ring ofpillars was completed, which took about 50 minutes beyond the firstpillar formation, the second latent pattern images formed just insidethe ring.

[0050] In a fashion similar to the formation of pillars in the firstring, a new pillar was formed at a new corner, then the adjacentcorners, later the new edges. As the process continued, pillar formationpropagated from the corners to the edges and from outside to inside (SeeFIG. 9h-l). Similar dynamic behavior has been observed in squarepatterns with different sizes as well as with mask patterns withdifferent shapes (e.g., triangles and rectangles). The atomic forcemicroscope image of the LISA pillar array shown in FIG. 10 shows thatthe diameter of each pillar is uniform and that the top of each pillaris substantially flat. The pillar height, diameter, and period is 310nm, 5 μm, and 9 μm, respectively. The measured height suggests that theactual mask-substrate spacing was 310 nm and that the spacer was pressed45 nm into the PMMA.

[0051] While not wishing to be bound by theory, FIG. 11 illustrates aproposed model for the formation of periodic supramolecular pillararrays in a film utilizing the LISA process. LISA appears to occur infour stages. The first stage is the surface roughening shown in FIG.11a. When a polymer 133 is heated above its glass transitiontemperature, it becomes a deformable and/or viscous liquid than canflow. Since there is an open space between the polymer 133 and the mask135, the polymer 133 will flow and deform three-dimensionally to relievethe polymer film's internal stress and surface tension, roughening thesurface of the polymer film surface.

[0052] The second stage is the enhancement of the polymer surfaceroughening shown in FIG. 11b. Placing a mask 135, preferably adielectric mask polymer 133, on top of the PMMA can create a Van derWaals attractive force, which is long-range and inversely proportionalto a power of the distance between the film 133 and the mask 135. Thecloser to the mask 135, the larger the attractive force on the polymer133, making the film roughness grow until some polymer touches thesurface 147 of the mask.

[0053] The third stage is the pillar 149 formation shown in FIG. 1 ic.In order to minimize the total free energy, the low energy surface ofthe mask forces the polymer melt 133 to ball up on the mask surface 147,forming round pillars 149. On the other hand, the high energy surface ofthe substrate 131 always keeps its surface 148 covered with a thin layerof polymer 133. The thin film layer connects all polymer pillars 149,allowing a polymer mass flow between the pillars 149. The thin filmlayer also acts as a polymer reservoir, supplying polymer to the pillars149. The connectivity and the polymer mass-transfer equalize thepressure inside all pillars, and hence the pillar diameter. The finaldiameter of a pillar 149 also depends on the other forces applied to thepillar 149, as discussed in the next paragraph.

[0054] The fourth and final stage is the self-organization shown in FIG.11d. Initially, the polymer pillars 149 have random locations andvarious diameters, and can move around inside the area defined by a maskpattern 137. But, once a pillar 149 moves to an edge of the mask pattern137, part of its triple-phase line (i.e. the line that intersects theliquid, solid and vapor phase) is trapped to the edge, limiting thepillar's movement to only along the edge. When a pillar 149 reaches acorner of the mask pattern 137, another part of its triple-line istrapped by another edge. Then that pillar cannot move any more, trappedat the corner, because a pillar 149 cannot move in two differentdirections at the same time; breaking away from one of the edgesrequires extra energy and is unlikely. Once pillars occupy the corners,other pillars start to self-organize on the edges. When theself-organization on the edges finishes, the self-organization ofpillars propagates gradually into the center of the mask pattern 137.During the cooling process, the polymer pillars solidify and maintainthe self-assembled patterns.

[0055] It appears the self-organization of pillars is due to the balancebetween long-range attractive force and the short-range repulsive force.The attractive force brings the pillars close together, while therepulsive force keeps the pillars apart. The cross-over of the twoforces fixes the distance between the pillars. This is similar to theself-organization in colloids. We believe that the surface of the PMMAin this case has like-charges. Therefore, the pillars appear to beattractive when they are a certain distance away, but repulsive when thepillars are very close. The attractive force between like-charges couldbe induced by the substrate and mask, similar to the situation of twomicrospheres between two glass walls. In the self-organization stage(the fourth stage), the pillar diameter continues to adjust to balancethe surface tension, the repulsive force and the attractive force. Sincethe pillars at corners have less repulsive force than those in thecenter, the diameter of the corner pillars is slightly bigger, asobserved in our experiments.

[0056] From the above observations and others, it appears that the LISAprocess is related to (i) a long-range attractive force between thepolymer melt film and the mask, (ii) the hydrodynamic forces in thepolymer melt, (iii) the surface tension, and (iv) the interplay of allthe forces. The long range force could be electrostatic, dipole, or Vander Waals forces, or a combination of all. It appears that electrostaticforce is the driving force. The patterns are formed as a result ofinterplay and instability of charges in a polymer melt, image charges ina mask, and hydrodynamic force in the polymer melt. We observed thatbecause the polymer melt thickness is ultra-thin, LISA is not due to theinstabilities from the thermal convection (Rayleigh-Benard instability)or the surface tension driven Benard convention, which also could leadto the pattern formation.

[0057] If there is no mask placed on top of the PMMA melt, the chargesin the PMMA film should be uniformly distributed due to a flat surfaceand symmetry. However, if there is a mask with a finite conductivityplaced near the PMMA melt, an image charge will be induced in the mask.The interplay of the charges and the image charges can cause instabilityand pattern formations. Again, not wishing to be bound by theory, weconsider the case that the mask has a protruding square. Since thecharge tends to accumulate at corners, there will be more image chargein the corners than other places on the protruding square, causing anonuniform charge distribution in the mask. The nonuniform distributionof the image charge will cause redistribution of the charges in PMMAfilm. The process continues in a positive feedback fashion. Eventually,enough charges and image charges will build up at the corners of thesquare mask pattern and in the PMMA areas under the corners, so that theelectrostatic force between the corners of the mask patterns and thePMMA under the corners exceeds the gravitational force. The PMMA melt inthose areas, which initially were flat, starts to be pulled up intosmaller cones. The charge will move into the sharp point of the cones,hence inducing more image charges at the corners of the mask. If themask is not too far away from the PMMA, the charges and the imagecharges will build up a local electric field, that can overcome thesurface tension. In this case, the small PMMA cones will grow. Thegrowth will reduce the distance between the charges and the imagecharges, hence increasing the strength of the electrostatic force andspeeding up the growth. Finally the PMMA pillars reach the mask, forminga full pillar. Once the full PMMA pillars are formed at the corners, thecharges and image charges must redistribute. The pillars formed become aboundary for the capillary waves in the PMMA melt surface. The capillarywave, a linear wave of amplitude of about one-hundredth of the filmthickness (less than 1 nm in our case) will form standing waves set bythe boundary. If the standing wave peak next to a boundary pillar has anamplitude slightly larger than the rest of the peaks, more charges willbe accumulated in that peak and more image charges in the mask areaabove the peak, making the peak grow into a full pillar. Once thepillars reach the mask, the process will repeat, until all smallamplitude capillary peaks under the mask protruding patterns developinto full pillars.

[0058] Therefore, the formation of the PMMA pillars starts at thecorners, then the edges, and later propagates into the center of themask protruding pattern. On the other hand, the polymer under the recessareas of the mask is too far away to have an electrostatic force toovercome surface tension to develop into full pillars.

[0059] The protruding patterns on the mask guide the boundary of thepillar array. The pillar array has a size, shape and period, that areriot only different, but smaller than the features on the mask. Suchdemagnification is technologically significant and could be usedrepeatedly to achieve even smaller patterns. With a suitable set ofpolymers of desired properties (e.g., viscosity, surface tension, etc)and a repeated usage of LISA, the diameter of the pillars can be“demagnified.” Furthermore, LISA showed for the first time the role oftrapping the three-phase lines by a mask pattern in self-organization ofa polymer melt.

[0060] The LISA process would appear to be applicable to other polymersand materials, especially single-phase materials, such assemiconductors, metals, and biological materials. The periodic arraysformed by LISA have many applications, such as memory devices, photonicmaterials, new biological materials, just to name a few. With a properdesign, a single crystal lattice of pillar array with predetermineddiameter, period, location, and orientation could be achieved over anentire wafer.

[0061] Utilizing the principles elucidated in LISA, we have been able tocontrol the surface energy and form patterns with a size identical tothe patterns on the mask can be formed. We refer to this aslithographically-induced self-construction (LISC). It differs from LISAin that the relief pattern is substantially identical in lateraldimensions to the patterns on the mask as opposed to the pillar arraysfound in LISA. LISC offers a unique way to pattern polymer electronicand optoelectronic devices directly without using the detrimentalphotolithography process.

[0062] As can be seen in FIG. 12, in LISC, a mask 235 with a protrudedpattern 237 is placed a certain distance above an initially flat polymer233 that is cast on a substrate 231. During a heating process thatraises the temperature above the polymer's glass transition temperatureand during cooling back to the room temperature, the polymer wasattracted, against gravitational force and surface tension, to maskprotrusions 237, but not to the recess areas of the mask, forming thepolymer mesas 249 on their own. The mesas have a lateral dimensionsubstantially identical to the protruded patterns on the mask 235, aheight equal to the distance between the mask 235 and the substrate, anda relatively steep sidewall.

[0063] In the LISC experiments, both the mask and the substrate are madeof silicon. The protrusions with a variety of shapes have a height of−300 nm. The polymer is polymethal methalcrylate (PMMA) which wasspin-cast on the substrate and was baked at 80° C. to drive out thesolvent. The molecular weight and thickness is typically 2000 and 100nm, respectively. The gap between the initially flat polymer film andthe mask protrusions ranged from 100 to 400 nm, and was controlled by aspacer. The temperature was cycled from room temperature to 170° C. Theheat was from the top and bottom of the sample, making the thermalgradient on the sample very small. A press was used to supply the heatand to hold the gap constant. A surfactant with a low surface energy wascoated on the mask to facilitate the mask-sample separation after LISC.We found that the materials (for the mask and substrate) and theparameters (e.g., the protrusion height, polymer thickness, polymersmolecular weight, gap, etc.) are not very critical to LISC. LISC can beformed over a wide range of these parameters. The typical diameter ofthe masks and substrate is larger than 2 cm. The masks are made byphotolighography and etching. The temperature was cycled to 175° C. Atpresent, the minimum size of the polymer microstructures formed by LISCis limited by the patterns on the mask. However, the demagnificationeffect observed in LISA could be used to form a resist wherein thesubstrate is etched in the recessed areas of the pattern to thereby formsmaller and smaller patterns on masks.

[0064] To further test the ability of LISC in forming an arbitrarypattern, we again created the protrusions of the word “PRINCETON” on aLISC mask. This is shown in FIG. 13, comparison of PMMA LISC patternswith the mask showed that the polymer mesas formed in LISC duplicate thepatterns on the mask very well. The linewidth and the height of thepattern are 3 μm and 230 nm, respectively. The initial PMMA filmthickness is only 100 nm.

[0065] In LISA, an array of periodic polymer pillars was formed under asingle mask protrusion, instead of a single polymer mesa with the samelateral dimension as the mask protrusion is formed as in LISC. The keyto have a LISC rather than LISA appears to reduce the difference of thesurface tensions of the polymer and the mask. When the difference issmall enough, each polymer pillar formed in the initial phase of LISCwill spread and merge with other pillars to form a single mesa undereach mask protrusion. Either using a different surfactant on the mask orincreasing the processing temperature (which would reduce the polymersurface tension) can reduce the surface tension difference.

[0066] Another embodiment of the present invention is self-alignedself-assembly (SALSA) of random access electronic device arrays. Theconventional approach in fabricating such an array is to fabricate eachindividual device first, then connect them with word lines and bitlines. As the devices become smaller, the precision alignment betweenthe wires and devices becomes more difficult to fabricate, substantiallyincreasing the fabrication cost. Using the LISA principle, we can firstfabricate a word-line array and a bit-line array in two differentsubstrates, and then let the device self-assemble between the word-lineand bit-line. FIG. 14 illustrates the applicability of SALSA to RandomAccess Electronic Devices. A word line assembly is fabricated utilizinga silicon wafer as is known in the art (e.g., by acid etching).Similarly, a bit line is fabricated with a silicon wafer as is known inthe art. A thin film or polymer 73 (e.g., PMMA) is deposited on the wordline assembly 77. The bit line 75 is placed a pre-determined distanceabove the word line 77 or vice versa, e.g. a distance of less than 1micron and preferably in the range of about 100 to 400 nm. Thetemperature is cycled from room temperature to the glass transitiontemperature of the polymer 73 and then cooled back down, to thereby forma pillar structure at the juncture of each word/bit line.

[0067] Although the present invention has been described with referenceto preferred embodiments, one skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method of forming. a pattern on a surfacecomprising: placing a plate above a surface layer of a material;maintaining said plate above said surface of said material; allowingpattern formation to occur via interaction between said plate and saidsurface layer.
 2. The method of claim 1, wherein the step of allowingpattern formation to occur includes rendering said surface deformable.3. The method of claim 2, wherein said material is a polymer.
 4. Themethod of claim 3, wherein said polymer is rendered deformable byheating the polymer to the polymer's glass transition temperature. 5.The method of claim 1, wherein said material is a thin film deposited ona substrate.
 6. The method of claim 5, wherein the substrate is selectedfrom the group consisting of semiconductors, dielectrics, metals,polymers and combination thereof.
 7. The method of claim 1, wherein saidmaterial is selected from the group consisting of a homoploymer, acopolymer, a polymer blend, a liquid, a liquid polymer, liquid crystals,a semiconductor, a metal, and a dielectric material.
 8. The method ofclaim 1, wherein said pattern is comprised of a plurality of pillars. 9.The method of claim 8, wherein said plurality of pillars is in aperiodic array.
 10. A method for forming a pattern on a surface,comprising the steps of: obtaining a substrate; depositing a polymerfilm on the substrate; placing a mask above the film, said mask having aprotruding feature; and heating the polymer film to thereby form acontact between said film and said protruding feature.
 11. A method ofnanolithography comprising the steps of: depositing a material on asubstrate; placing a mask a distance above said material, maintainingsaid mask above said material, said material and substrate interactingto form a pattern in said material on said substrate.
 12. The method ofclaim 11, wherein the material comprises a thermoplastic polymer. 13.The method of claim 11, further including heating the material to saidmaterial's glass transition temperature.
 14. The method of claim 11,wherein the substrate is selected from the group consisting ofsemiconductors, dielectrics, metals, polymers and combination thereof.15. The method of claim 11, further including the step of removing saidmask after said pattern is formed.
 16. The method of claim 11, whereinsaid pattern is comprised of a plurality of pillars.
 17. The method ofclaim 16, wherein said plurality of pillars is formed as a periodicarray.
 18. A method of forming a relief pattern on a surface of amaterial composed of: positioning a mask a predetermined distance abovethe surface of the material; and altering the surface of the material toa deformable surface, said mask and said deformable surface interactingto form said relief pattern.
 19. The method of claim 18, wherein saidrelief pattern has a height of about 10 nm to about 1,000 nm.
 20. Themethod of claim 18, wherein said relief pattern has a height of about 50nm to about 750 nm.
 21. The method of claim 18, wherein said reliefpattern has a height of about 100 nm to about 700 nm.
 22. The method ofclaim 18, wherein said surface is altered by heating to a glasstransition temperature of said material.
 23. The method of claim 18,wherein said mask has a pattern formed thereon.
 24. The method of claim18, wherein said relief pattern is patterned after said pattern on saidmask.
 25. The method of claim 18, wherein said relief pattern iscomprised of a plurality of pillars.
 26. The method of claim 18, whereinsaid relief pattern has a height of less than about 1 μm.
 27. The methodof claim 18, further including the step of cooling said material aftersaid relief pattern is formed.
 28. The method of claim 18, wherein saidpredetermined distance is about 2 to about 7 times a thickness of saiddeformable surface of said material.
 29. The method of claim 28, whereinsaid deformable surface thickness is in a range of about 1 nm to about2,000 nm.
 30. The method of claim 29, wherein said deformable thicknessis in a range of about 5 nm to about 1,000 nm.
 31. The method of claim30, wherein said deformable thickness is in a range of about 50 nm toabout 500 nm.
 32. The method of claim 31, wherein said deformablethickness is in a range of about 75 nm to about 250 nm.
 33. The methodof claim 32, wherein said deformable thickness is about 100 nm.
 34. Themethod of claim 18, wherein said mask is dielectric.
 35. The method ofclaim 18, wherein said material is a viscous liquid.
 36. The method ofclaim 18, wherein said material is a polymer.
 37. The method of claim18, wherein said polymer is a homopolymer.
 38. A microscale patternforming assembly comprised of: a substrate; a material deposited on saidsubstrate; and a mask positioned a predetermined distance above saidmaterial.
 39. The microscale pattern forming assembly of claim 38,further including a spacer interposed between said material and saidmask to maintain said mask at said predetermined distance.
 40. Themicroscale pattern forming assembly of claim 39, wherein said mask has aprotruding pattern formed thereon.
 41. The microscale pattern formingassembly of claim 38, wherein said substrate has a higher glasstransition temperature than said material.
 42. The microscale patternforming assembly of claim 38, wherein said mask is dielectric.
 43. Themicroscale pattern forming assembly of claim 38, wherein said materialis a viscous liquid.
 44. The microscale pattern forming assembly ofclaim 38, wherein said material is a polymer.
 45. The microscale patternforming assembly of claim 38, wherein said mask has a pillar formed fromsaid material in contact therewith.
 46. The microscale pattern formingassembly of claim 38, wherein said mask and said material have aplurality of pillars formed there between.
 47. A method ofnanolithography comprising: depositing a material on a substrate;placing a mask a distance above said material, said mask havingprotrusion patterns formed thereon; and forming a pattern in thematerial corresponding to said protrusion patterns, said pattern being aresult of an interaction between said protrusion patterns and saidmaterial.
 48. The method of claim 47, wherein said protrusion patternsis comprised of a first protrusion pattern and a second protrusionpattern, said first and second protrusion pattern being of differentlength.
 49. The method of claim 47, wherein said method is coated with asurface coating.
 50. An article having nanoscale patterning, saidarticle being comprised of a plurality of pillars, said plurality ofpillars having a height ranging from above 1 nm to below 1 μm.
 51. Thearticle of claim 50, wherein said height is in the range of about 100 nmto about 700 nm.
 52. The article of claim 50, wherein said height is inthe range of about 250 nm to about 550 nm.
 53. The article of claim 50,wherein said pillar has a diameter, said pillar height to pillardiameter ratio being in a range of about 0.1 to about 0.5.
 54. Thearticle of claim 50, wherein said plurality of pillars are in a periodicarray.
 55. The article of claim 50, wherein said plurality of pillarshas a period of about 1 μm to about 10 μm.
 56. The article of claim 50,which said plurality of pillars has a boundary defined by a pattern on amask used to form said plurality of patterns.
 57. The article of claim50, wherein said plurality of pillars are connected to form alithographically-induced self-construction.
 58. The article of claim 50,wherein said nanoscale patterning is substantially identical in lateralsize as a mask used to form said nanoscale patterning.